This invention relates to a power coupling assembly for superconducting magnets, particularly suitable for connecting power to the shim magnets.
As is well known, a magnet can be made superconductive by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnet coils to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of ten minutes) to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter "MRI").
Shim magnets are auxiliary magnet coils provided proximate to the main magnet coils to adjust or "fine-tune" the magnetic field produced by the main magnets, even in the presence of surrounding magnetic structures and/or magnetic flux emitting devices. Unlike the leads to the main magnet coils, the shim magnet coils are typically permanently connected in order to enable subsequent adjustment of the magnetic field to obtain a uniform magnetic field even in the presence of variations in the magnetic environment surrounding the superconducting magnet.
In a typical superconducting magnet, the cryostat or pressurized helium vessel is positioned within, but spaced from, an outer shell or vacuum vessel with a significant temperature gradient therebetween. As a result, the shim lead power coupling assembly must extend through a wide temperature range from the outside of the outer shell to the pressurized vessel. There is a need to minimize the heat conducted through the shim leads and the shim lead assembly to the cryogen, since even a single watt of heating can result in the boiling of 1.4 liters/hour of helium, which is completely unacceptable since a typical MRI specification limits the helium boil-off to 0.2 liters/hour. As a result, the helium gas boiled off, which is extremely cold (in the order of -270.degree. C.), has been passed through the shim lead coupling assembly to remove a significant portion of the heat conducted down the wires.
The bare wire of the leads are provided with only a thin layer of insulation to enhance their heat transfer efficiency. However, thin insulation introduces the danger of shorts between adjacent leads. Moreover, a shim lead coupling assembly may typically include as many as thirty-six adjacent wires. Also, the diameter of the tube through which the leads pass must be minimized in order to minimize heat transfer through the tube, causing the leads to be placed close together, notwithstanding that the insulation must also be minimized in order to enhance heat transfer from the leads to the helium gas passing by. Thin Formvar insulating coatings can help provide needed removal of heat conducted down the leads but present problems of possible shorting between adjacent leads, particularly for current flow in excess of 10 amperes.
The shim lead power assembly design should optimize the electrical parameters by minimizing lead resistance and power consumption, along with heating, while at the same time preventing electrical shorts even in the presence of the differential expansion of materials in the presence of temperature cycles to which the assembly is subjected when placing it into operation, and during operation of the shim magnet coils. The thermal parameters must also be optimized to minimize heat conducted to the magnet. Moreover, heating due to current flow through the power coupling assembly must be minimized, not only to minimize the helium boil-off, but also to avoid quenching of the magnetic field after it is initially ramped to field.
As a result, conflicting thermal, electrical, magnetic and mechanical considerations and factors must be balanced and compromised in order to obtain an optimum design.